X-ray imaging is a valuable diagnostic tool for analysing the interior structure of a sample. In an x-ray imaging system, a sample is placed between an x-ray source and an x-ray detector, and the detector is irradiated with x-rays from the source via the sample. The intensity of the x-rays received at the detector via the sample depends on the degree to which the portions of the sample through which the x-ray beam passes attenuate the x-ray beam. Analysis of the detected x-rays is therefore able to provide information about the internal structure of the sample.
In traditional two-dimensional x-ray imaging, a single projective image of a sample is acquired by placing the sample between an x-ray source and a two-dimensional detector, or a photographic plate, to measure a single projection of the sample. If the dimensions of the source are small compared with the distance to the sample, each point on the detector receives x-ray radiation which has passed through the sample at a well-defined angle from the source. While useful in some circumstances, this technique provides only limited information about the internal structure of the sample, since radiodense (relatively opaque to x-ray) features of the sample will tend to occlude radiolucent (relatively translucent to x-ray) features, and information about the sample in the direction of the beam centreline, that is, depth information, is lacking.
In computed tomography (CT), a series of x-ray projections are acquired over a range of angles about one or more predetermined axes of the sample. This can be achieved by holding source and detector fixed in a predetermined spatial relationship and rotating the sample relative to the source and detector, or by holding the sample fixed and rotating the source and detector in a predetermined spatial relationship about the sample. At each angular step, one x-ray projection of the series is acquired. Mathematical reconstruction using each of the projections in the acquired series allows a volumetric map of the radiodensity of the interior of the sample to be obtained. Such a reconstruction can provide valuable diagnostic information about the interior of the sample.
However, in computed tomography, the acquisition process may involve acquiring projections at a series of angles about the sample with increments between angles in the series of as little as 0.1 or 0.05 degree. The smaller the angular interval between projections, the better the resolution of the volumetric reconstruction is. Also, the signal-to-noise in each of the acquired projections is generally improved with longer exposures for each projection. Therefore, a single computed tomography acquisition series can be very lengthy, if it is important to obtain either or both of high resolution and good signal-to-noise.
During the series acquisition, changes in, for example, the size and shape of the sample, the relative alignment of source and detector, or the relative separation of source, detector, and sample can result in blurring of the reconstructed volume map and a consequent loss of resolution. It is thus of importance to ensure that, aside from the intended incremental change in angle of the x-ray projection about the sample, all other dimensions and spatial relationships of the sample and the x-ray imaging system remain constant throughout the imaging process.
Therefore, in high-resolution imaging systems, it is important that the dimensional stability of the whole system, including the dimensions of the source, the detector and the sample, as well as their relative separations and orientations, is preserved even over long timescales.
It is conventional that the x-ray imaging system is contained within an enclosure which is opaque to x-rays. This allows an operator to conveniently operate the x-ray imaging apparatus, as well to make appropriate adjustments to the sample or exchanges of one sample with another, between accumulations, without being subject to the x-ray radiation. When such an enclosure is in a closed state, the temperature inside the enclosure may be well-controlled. Therefore, dimensional stability of both the sample and the source-detector system, which comprises the source and detector arranged in a predetermined spatial relationship, may be high. Specifically, the dimensional stability of the source-detector system can include the stability of the alignment between the source and the detector, the stability of the spacing between the source and the detector, and the stability of the dimensions of individual components of each of the source and the detector, at least.
However, when the enclosure is opened, either to adjust the position of the sample or to replace the sample with another sample, the temperature of the air inside the enclosure can change depending on the temperature of the air outside the enclosure. As a consequence, thermal expansion, or contraction of the sample and of the source-detector system can occur.
Once the enclosure is closed, the temperature inside the enclosure will return to a normal operating condition over a period of time. During this period of time, any x-ray accumulation will tend to suffer blurring defects due to thermal expansion or contraction of the source-detector system, of the sample, or of both.
Therefore, it is customary to wait for the enclosure and sample temperatures to stabilise after the enclosure is closed and before the accumulation of imaging data begins. This limits the rate at which a series of different samples may be imaged, and also lengthens the imaging process if, for example, the position or attitude of the sample requires adjustment between exposures. In particular, to obtain a particular view of a portion of a sample, an operator may need to manually adjust the position of the sample several times, acquiring an image at each time, before the desired view is obtained. The need to allow time for the temperature inside the enclosure to stabilise slows this procedure.
There is therefore a need for an enclosed x-ray imaging system which is better able to maintain the internal temperature even when the enclosure is opened.